This invention relates to turbine bucket construction and, more particularly, to the addition of a rib in the cavity of a cored turbine bucket for altering the bucket""s natural frequencies.
Gas turbine buckets (blades) operate in an environment where they may be stimulated by multiple impulses, which in turn drive responses corresponding to various natural frequencies of the bucket. The buckets also operate over a variety of speed ranges as well as, at a given speed, different sources of stimuli, exposing them to a large variety of stimuli. It is important to avoid the crossing of a driving stimulus and the bucket natural frequency to prevent premature failure of the bucket in high cycle fatigue. Often, the design of the bucket in terms of its aerodynamic shape, internal cooling geometry, and the like, is dictated to avoid such crossings.
Previously, turbine bucket tuning has been accomplished using devices such as altering the blade aspect ratio (height to chord), TE (trailing edge) cropping, changes in camber, wall thickness, tip mass, shank height, damper designs, and material density or other material properties (e.g., DS, mono-crystal), etc.
It would be desirable, however, to alter certain natural frequencies of a gas turbine bucket so as to avoid these detrimental crossings of natural frequencies and stimuli without impacting other features that are important to the performance of the gas turbine to thereby improve the reliability of a gas turbine.
In an exemplary embodiment of the invention, a method of tuning a turbine bucket having an internal cavity includes (a) designing the turbine bucket construction, (b) testing the turbine bucket, and (c) after steps (a) and (b), adding a rib in the internal cavity to thereby alter a natural frequency of the turbine bucket. Step (c) may be practiced by adding a rib in an aft cavity of the turbine bucket to stiffen the compliant trailing edge. The rib may be solid or segmented. This construction is particularly suited for altering high order frequency modes such as 2T, 4F and 1-3S.
In an another exemplary embodiment of the invention, a turbine bucket includes an internal cavity and a tuning rib added in the cavity that alters a natural frequency of the turbine bucket.